Hey Guys I need some help with picking and finding the correct speedo gear. I have a buick 455 with a th400 in a 68 skylark and have updated to a 3.36 gear ratio in the rear. I am unsure of which gear to get to make my speedometer work correctly. Please help if you can. thank you My tire height is 26.5 inches My wheels are 20" My rear gears are 3.36 My tire width is 9.3" My diameter is 83.1
You will nred to know tire height b4 any help can be given on this suject. I always just call a reputable trans shop and they send me what I need. I use ck performance
My tire height is 26.5 inches My wheels are 20" My rear gears are 3.36 My tire width is 9.3" My diameter is 83.1
None of that matters. What is the ROLLING DIAMETER? This is different from the tire diameter because the tire flattens at the bottom when weight is on the tire. Most folks just figure out true speed vs. indicated speed, and select speedo gears based on the amount of correction needed.
How does the rolling circumference change just because the tire flattens at the contact patch? If you figure out the circumference from the tire height fully inflated, that will be the circumference of the tire which is what really matters. I have had great results using the formula from the TCI site. Very accurate according to my GPS anyway.
Use the link above. It helped me calibrate my speedo perfectly. I verified it by going a known mile at 60 (indicated) and it took 60 seconds with a stop watch. We repeated several times.
The rolling circumference of the tire is always smaller than the "true" circumference. Rolling circumference is based on the radius from center of hub to ground, not the center of hub to the fully-rounded upper part of the tire.
Can't agree with that. The tire is a circle. Circumference is Pi X Diameter. Circumference does not change because of the small deflection of the contact patch. The formula uses tire revolutions per mile. It is very close, at least close enough. The error if there is one is trivial for what we are trying to do.
The tire with weight on it is not a circle. It's a distorted circle. Yes, not correcting for rolling circumference may produce "close-enough" results. Nobody corrects for slippage, either.
I understand that, I just don't see how the circumference of the tire changes from that. Does it shrink?
Yes. By the amount that the "circle" is distorted. Consider the opposite: Drag slicks at high speed "grow" in radius/diameter/circumference, changing the effective gearing of the car. In both cases, what's important is the distance from hub center to the pavement--the rolling radius, from which we can compute the rolling circumference. Circumference of the tire can be a static, "no load, no speed" measurement, but the important measurement is the dynamic "in use" measurement.
First from a stop with a marker landmark or someting. Use a good gps. Reset or know ware your tripometer is on. Drive until u have gone one mile. And stop. Look at your trip or mileometer. If you havent gone a mileyou need spedo gears with a higher num on them. If milometer puts you past a mile youl need lower num on your spedo gears.
On a round gear ratio calculater you turn the thing line up your tire size gear ratio rpm and it will tell you wat ur speed is. Cars with rubber band tires dont compress much at all. Its not needed to Break It Down Scientificly and input a half of a half of a half of an inch less tire measurment.
